Application device

ABSTRACT

A device ( 10 ) for the direct or indirect application of liquid or pasty application medium ( 16 ) to one or both sides of a material web ( 20 ), in particular of paper or board, comprises an applicator unit ( 12 ) which discharges the application medium ( 16 ) onto the material web ( 20 ) in the form of a free application medium jet ( 18 ). In this case, in the region of the applicator unit ( 12 ) there is provided a device ( 30 ) for producing an electric field, which exerts on the application medium jet ( 18 ) moving from the applicator unit ( 12 ) to the material web ( 20 ) a force which assists its movement. Additionally or alternatively, edge guiding elements can be provided, which guide the lateral edges of the application medium curtain ( 18 ) at least on part of its movement under the force of gravity.

DESCRIPTION

[0001] The invention relates to a device for applying liquid or pastyapplication medium to one or both sides of a moving substrate,comprising an applicator unit which is arranged at a distance from thesubstrate and discharges the application medium onto the substrate inthe form of a free application medium jet, the substrate, in the case ofdirect application, being the surface of a material web, in particularof paper or board and, in the case of indirect application, being thesurface of a transfer element, preferably a transfer roll, which thentransfers the application medium to the surface of the material web.

[0002] Although the invention can also advantageously be used in otherapplication devices which operate “without contact”, for exampleapplication devices having a free jet nozzle applicator unit, whichdischarges the application medium onto the substrate as an applicationmedium jet which moves from the applicator unit to the substratesubstantially on account of the expulsion momentum imparted to it by theapplicator unit, the invention will be explained in more detail belowusing the example of a curtain application device, that is to say anapplication device in which the applicator unit discharges theapplication medium onto the substrate as a curtain or veil which movessubstantially under the force of gravity.

[0003] In the coating of material webs by using a curtain applicatorunit (also known as “curtain coating” in the specialist world), theapplication medium is discharged onto the substrate in the form of anapplication medium curtain, which moves from the applicator unit to thesubstrate substantially under the force of gravity. The fact that inthis case the curtain applicator unit is located at a predetermineddistance from the substrate has, inter alia, the advantage that it isexposed to a low risk of damage, for example in the event of a webbreak. Curtain applicator units differ fundamentally from other“non-contact” applicator units, for example free jet nozzle applicatorunits, in which the movement of the application medium from theapplicator unit to the substrate is brought about primarily by theexpulsion momentum from the discharge nozzle of the applicator unit,since the shape of the curtain emerging from the discharge nozzle isexposed only to the interplay between the surface tension of theapplication medium and the force of gravity. In this case, surfacetension attempts to contract the curtain which, in relation to itsvolume or its cross-sectional area, has a very large surface orcircumferential length, in order in this way to reduce its surface. Thiseffect is opposed only by the force of gravity, which attempts tostretch the curtain. It can therefore easily be seen that it is all themore difficult to obtain an application medium curtain which isuniformly thick over the entire working width, the greater this workingwidth is.

[0004] The coating of material webs by means of a curtain applicatorunit, which supplies the material web with the application medium as anapplication medium curtain or veil that moves substantially under theforce of gravity, has been known for a long time from the coating ofphotographic films, audio tapes and the like. However, the material websin these areas of application have a considerably lower width than isthe case in modern installations for the production of paper andpaperboard webs, in which material web widths of more than 10 m arerequired. To be able to form an application medium curtain which isuniformly thick over this width and to keep it stable is a task in whichit is anything but obvious to expect suggestions for a working solutionfrom the comparatively simply controlled, known narrow applicationmedium curtains. Furthermore, in modern installations for the productionof paper and paperboard webs, the material webs move at speeds of up to3 000 m/min, which is many times the speed at which the known narrowmaterial webs move and, furthermore, constitutes a further high loadingon the stability of the application medium curtain.

[0005] DE 199 03 559 A1 presents a whole series of principles of actionwhich are intended to permit the air boundary layer carried along by thematerial web to be attenuated immediately upstream of a curtainapplicator unit. However, this document does not discuss the possibleways of improving the efficiency of these principles of action.

[0006] WO 97/03009 tackles the problem of the drying of material websfollowing the application of media, specifically printing inks, inparticular in gravure, web-fed offset and flexographic printing. Itproposes to ionize the gas molecules on the surface of the material webby means of a corona discharge and to accelerate them toward anelectrode, in order to increase the drying efficiency by the gasexchange at the material web surface which is associated with this “ionwind”.

[0007] For completeness, reference should further be made to DE 198 03240 A1 and DE 198 29 449 A1 in relation to the further prior art.

[0008] By contrast, it is an object of the present invention to furtherimprove application devices which operate “without contact” for use ininstallations for the production and/or finishing of wide andfast-moving material webs, preferably of paper or board, in particularin relation to the stabilization of the application medium jet orcurtain.

[0009] According to the invention, this object is achieved by a devicefor applying liquid or pasty application medium to one or both sides ofa moving substrate, comprising an applicator unit which is arranged at adistance from the substrate and which discharges the application mediumonto the substrate in the form of a free application medium jet, thesubstrate, in the case of direct application, being the surface of amaterial web, in particular of paper or board, and, in the case ofindirect application, being the surface of a transfer element,preferably a transfer roll, which then transfers the application mediumto the surface of the material web, and in the region of the applicatorunit there being provided a device for producing an electric field,which exerts on the application medium jet moving from the applicatorunit to the substrate a force which assists its movement

[0010] As distinct from the above-described conventional “contactless”application devices, in particular curtain application devices, in thecase of the application device according to the invention theapplication medium jet does not move from the discharge nozzle to thesubstrate solely under the influence of the force of gravity or solelyunder the influence of the expulsion momentum. Instead, this movement isassisted by an additional force which is exerted on the applicationmedium jet by the field production device and attempts to stretch thejet on its path from the discharge nozzle to the substrate(pre-stretching). Therefore, in particular in “curtain coatingapplicator units”, either the stability of the jet can be increased withthe same distance between discharge nozzle and substrate or the distancebetween discharge nozzle and substrate can be chosen to be greater witha predefined desired jet stability.

[0011] The latter is advantageous in particular if it is considered thatthe total stretching of the application medium from a jet thicknessdetermined by the width of the discharge nozzle down to the thickness ofthe layer of application medium ultimately applied to the substrate iscomposed of the product of the above-described pre-stretching of the jeton the path from the discharge nozzle to the substrate and the contactstretching, which is brought about by the difference between the speedof the jet immediately before it strikes the substrate and the speed ofmovement of the substrate at the point of contact between: theapplication medium and the substrate. To achieve the most uniformapplication layer possible, it is specifically advantageous if the totalstretch is composed as uniformly as possible of the pre-stretch andcontact stretch. Since the application medium jet is all the moresusceptible to external influences, for example to the influence of theair boundary layer carried along by the substrate, the longer the pathof the application medium from the discharge nozzle to the substrate, inpractice there is always the desire for the most intense pre-stretchpossible.

[0012] Furthermore, the electric field also exerts on the applicationmedium jet a force which has a component extending orthogonally to itsdirection of movement, that is to say the electrode arrangement attractsthe application medium jet. This force component also helps to stabilizethe application medium jet with respect to the influence of the airboundary layer carried along on the surface of the substrate.

[0013] The electric field that assists the force of gravity inaccelerating the application medium jet can be produced in astraightforward manner by the applicator unit being kept at a firstpredetermined electric potential, while the substrate can be kept at asecond predetermined electric potential, for example ground or earthpotential. In this case, no additional electrodes need to be provided;instead it is sufficient to provide the existing devices with electricalconnections.

[0014] Additionally or alternatively, however, an electrode arrangementcan be provided on the upstream side of the applicator unit and in itsvicinity, which is kept at a third predetermined electric potential. Inthis case, “additionally” means that both the applicator unit and theelectrode arrangement are kept at a predetermined potential differentfrom ground potential. On the other hand, “alternatively” means thatonly the electrode arrangement are kept at a predetermined potentialdifferent from ground potential, while the applicator unit is eithergrounded or floats with regard to its electric potential (“floatingpotential”).

[0015] The electrode arrangement can comprise at least one flatelectrode. In this case, the flat electrode can have a plurality ofprojections or needle points on its side pointing toward the substrate.Alternatively, however, it is also possible for the electrodearrangement to comprise a plurality of individual electrodes, preferablyneedle electrodes, arranged adjacent to one another in the transversedirection of the substrate. The use of needle points or needleelectrodes means that the electrode arrangement is influenced not onlyby the electric field formed with respect to the ground potential butthat, in addition, it is also possible for discharge processes to occur,during which the surrounding air is at least partly ionized, and thecharge carriers formed in this way lead to the application medium jet,for example, being charged up. Following charging of this kind, themovement of the application medium jet can be influenced still moreeffectively by the electric field.

[0016] In a development of the invention, it is proposed that a devicefor producing an electric field be provided downstream of the point atwhich the application medium strikes the substrate. This electric fieldexerts on the application medium applied to the substrate a force whichis directed toward the substrate and which, at locations at which toomuch application medium has been applied, leads to the excessapplication medium being displaced. In this way, firstly applicationmedium is supplied to locations on the substrate which have hitherto notbeen covered, which improves the covering of the substrate withapplication medium. Secondly, the thickness of the application layer canbe evened out in this way.

[0017] The further field producing device preferably has a furtherelectrode arrangement which is adjacent to the substrate and which ispreferably kept at a fourth predetermined electric potential.

[0018] However, such a force attracting the application medium appliedto the substrate toward the substrate can in principle also result fromresidual charging of the application medium applied to the substrate, asa result of discharges induced by the first electrode arrangement.

[0019] The predetermined electric potential or potentials that is/aredifferent from ground potential can, for example, have a value ofbetween 5 kV and about 60 kV, preferably about 30 kV.

[0020] As already mentioned above, the substrate is preferably kept atground potential. This can be implemented, for example, by a backingelement, preferably a backing roll, which, in the case of directapplication, supports the material web in the region of the applicatorunit, or, in the case of indirect application, on the surface of whichthe applicator unit applies the application medium, being in contactwith an electrode in order to keep said roll at the second predeterminedelectric potential. Additionally or alternatively, however, provisioncan also be made for the surface of the backing element, which ismetallic or rubber-covered, for example, to be in wiping contact withthe electrode. Finally, it is also further possible for the electrode tobe in electrically conductive contact with a bearing shaft of thebacking roll. In the case of direct application, a further alternativeis added to this in that the material web is kept at the secondpredetermined electric potential, that is to say preferably groundpotential, by means of an electrode formed, for example, as a web guideelement.

[0021] In addition to the use of electric fields for influencing theapplication medium jet, other types of force fields are of courseconsidered, for example magnetic fields, if the application medium inany case contains as a constituent part particles which respond to amagnetic field, or these particles can be added to the applicationmedium for this purpose.

[0022] In order to be able to keep low the influence on the applicationmedium jet of the air boundary layer carried along on the surface of thesubstrate, it is proposed that, in the direction of movement of thesubstrate, a device for attenuating the air boundary layer carried alongby the substrate be arranged upstream of the electrode arrangement. Inthis case, the air boundary layer attenuation device can comprise asuction device, with the aid of which the air boundary layer can beremoved actively from the moving substrate.

[0023] For example, a trailing scraper in wiping contact with thesubstrate can be provided at the downstream end of the suction device.This trailing scraper seals off the suction device with respect to thesurroundings and prevents the onward movement of the air boundary layertoward the applicator unit. In this way, the air carried along in theair boundary layer is backed up, which at least partly destroys thelaminar character of the flow of the air boundary layer. This makes iteasier to extract and increases the suction efficiency of the suctiondevice. Thus, the air boundary layer can be attenuated particularlyeffectively, if not even completely removed from the substrate, by thesuction device designed in accordance with the invention.

[0024] The trailing scraper can be constructed as a flexible foil,preferably made of plastic, metal sheet or a composite material. Theflexible foil nestles against the substrate under the suction action ofthe suction device, which firstly improves the sealing and secondlyprevents the formation of a new air boundary layer. If the trailingscraper is fabricated from metal sheet, then use is preferably made ofstainless steel sheet with a thickness of at most 0.1 mm. However,trailing scrapers made of composite material with a surface coating ofTeflon have also proven to be advantageous. In this case, the compositematerial ensures the necessary temperature resistance and flexibility,while the Teflon surface coating ensures low friction between thetrailing scraper and the moving substrate. Furthermore, the trailingscraper can be curved in the direction of movement, which facilitatesits resilient compliance and further reduces the friction with thesubstrate.

[0025] In a development of the invention, it is proposed that in theregion of the suction device, preferably between the downstream end ofthe suction device and the trailing scraper a further electrodearrangement be provided. With the aid of this further electrodearrangement, the air boundary layer carried along by the substrate canalso be influenced and, in particular, attenuated. Above all, if thefurther electrode arrangement comprises a plurality of individualelectrodes, preferably needle electrodes, arranged adjacent to oneanother in the transverse direction of the substrate, or if the furtherelectrode arrangement comprises at least one flat electrode which has aplurality of projections or needle points on its side pointing towardthe substrate, it is likewise possible for discharge processes to occurbetween the points of the further electrode arrangement and thesubstrate. These discharges disrupt the laminar disruption of the airboundary layer and convert the latter at least partly into a turbulentflow, which makes it easier to extract the air boundary layer andtherefore further improves the effectiveness of the suction device. Inorder to assist the production of these discharges, it is proposed thatthe further electrode arrangement have a distance of between about 2 mmand about 30 mm from the substrate.

[0026] In principle, the further electrode arrangement can be connectedto an external voltage supply. However, it is likewise possible, andeven simpler to implement in constructional terms, if the electricpotential of the further electrode arrangement is kept floating(“floating potential”). In this case, the further electrode arrangementis charged up as a result of the discharges originating from the firstelectrode arrangement, and is therefore likewise brought to a potentialdifferent from ground potential. In order to reduce the capacitance ofthe further electrode arrangement, and also for reasons of safety, it isproposed that the further electrode arrangement be arranged on thesuction device electrically insulated from the latter.

[0027] According to a further point of view, the object of the inventionis achieved by a device for applying liquid or pasty application mediumto one or both sides of a moving substrate, comprising a curtainapplicator unit which discharges the application medium onto thesubstrate as a curtain or veil moving substantially under the force ofgravity, the substrate, in the case of direct application, being thesurface of a material web, in particular of paper or board, and, in thecase of indirect application, being the surface of a transfer element,preferably of a transfer roll, which then transfers the applicationmedium to the surface of the material web, edge guiding elements beingprovided, which guide the lateral edges of the application mediumcurtain at least on part of its movement under the force of gravitybetween the curtain applicator unit and the substrate.

[0028] As already mentioned above, the surface tension of theapplication medium attempts to reduce the surface of the applicationmedium jet, which has an effect in particular in the region of the sideedges of the application medium jet and leads to jet contraction. Thisjet contraction can be prevented by the provision of edge guidingelements since, in this case, adhesion forces additionally act betweenthe application medium and the surface of the edge guiding elements andoppose the tendency to contraction originating from the surface tensionof the application medium. The adhesion between the edge guidingelements and the application medium can be improved by the surfacecharacteristics of at least one edge guiding element being chosen suchthat the wetting or edge angle which depends on the characteristics ofthe application medium and the surface of the edge guiding elements isless than 90°. Furthermore, it is advantageous for the wetting of theedge guiding elements if the latter have a structured surface. For thispurpose, the surface of at Least one edge guiding element can beroughened and/or have a toothed surface, for example in the form of anexternal thread or else an internal thread.

[0029] The edge guiding elements can be fabricated from glass or metal,for example, since these materials have a sufficiently high surfacetension. In the case of metals, however, it must be noted that somemetals have the tendency to adsorb water vapor from the atmospheresurrounding them, as a result of which their surface tension decreasesto the value of the adsorbed water layer.

[0030] In order to be able to adapt the application device to the widthof the material web to be coated, it is proposed that at least one edgeguiding element be arranged such that it can be displaced in thetransverse direction of the substrate, and/or that the angle which anedge guiding element forms with the vertical be adjustable. Furthermore,the ability to adjust the angle can include pivoting the free ends ofthe edge guiding elements in the transverse direction and/or in thelongitudinal direction.

[0031] Finally, the action of the edge guiding elements can also beimproved by providing an electric field, for example an electrode beingprovided in the vicinity of at least one of the edge guiding elements,preferably extending substantially parallel to the latter, and beingkept at a predetermined electric potential.

[0032] The invention will be explained in more detail below using anexemplary embodiment and the appended drawing, in which:

[0033]FIG. 1. shows a rough schematic side view to explain theconstruction and function of an application device according to theinvention;

[0034]FIG. 2 shows a schematic view to explain the construction andfunction of the edge guiding elements; and

[0035]FIG. 3 shows an illustration to explain the term “wetting angle”or “edge angle”.

[0036] An application device according to the invention is designatedgenerally by 10 in FIG. 1. It comprises a curtain applicator unit 12having a discharge nozzle 14, from which the application medium 16 isdischarged in the form of a curtain 18 onto a substrate U moving in thedirection of movement L. In the exemplary embodiment illustrated, thesubstrate U is the surface 20 a of a material web 20 which, in theregion of the applicator unit 12, wraps partly around thecircumferential surfaces of a supporting roll 22.

[0037] On its path from the discharge nozzle 14 to the substrate U, theapplication medium 16 is accelerated. This leads to a reduction in thethickness of the curtain 18 from the value D in the region of thedischarge nozzle 14 to the value d immediately upstream of the positionP at which it strikes the substrate U (pre-stretch). Because of thedifference between the speed of the application medium curtain 18immediately before the striking position P and the speed of movement ofthe substrate U, the application medium 16 is again stretched when itstrikes the substrate U, so that the application layer 24 applied to thesubstrate U ultimately has the thickness s (contact stretch). The totalstretch of the application medium 16 is given by the product of thestretch factors of pre-stretch and contact stretch.

[0038] In practice, the application medium 16 is generally stretched toa greater extent when it strikes the substrate U than on the path fromthe discharge nozzle 14 to the substrate U, since the falling distancebetween discharge nozzle 14 and substrate U cannot be selected to bearbitrarily large, with a view to the most stable curtain 18 possible.However, an excessively high contact stretch has a detrimental effect onthe uniformity of the application layer 24 applied to the substrate U.

[0039] The application device 10 according to the invention now providesa possible way with the aid of which either with the same falling heightbetween discharge nozzle 14 and substrate U the pre-stretch of thecurtain 18 can be intensified or, given the same pre-stretch, thefalling height between discharge nozzle 14 and substrate U can bereduced and therefore the curtain 18 can be stabilized. To be specific,according to the invention, the application medium curtain 18 is notleft solely to the force of gravity on its path from the dischargenozzle 14 to the substrate U, but instead electrostatic forces areadditionally exerted on it. This intensification of the forcesstretching the curtain 18 results in higher stability of the applicationmedium curtain 18, since the surface tension of the application medium16, which attempts to contract the curtain 18 with the effect ofreducing its surface, remains constant.

[0040] In principle, the electrostatic forces could be provided by afirst specific electric voltage V₁ being applied to the housing of theapplicator unit 12 and the supporting roll 22 being kept at earth orground potential V_(E) or V₂. In the exemplary embodiment according toFIG. 1, however, another route is followed, which is to be explained inmore detail below:

[0041] According to FIG. 1, an electrode arrangement 30 is providedimmediately upstream of the applicator unit 12 in the direction ofmovement L and, for example, can comprise a flat electrode extending inthe transverse direction Q and having a plurality of needle points, orcan be formed by a plurality of needle electrodes 32 arranged adjacentto one another in the transverse direction Q. A predetermined electricvoltage V₃ is applied to the electrode arrangement 30, while the partsof the application device 10 which surround it, namely the applicatorunit 12, the supporting roll 22 and a suction box 34 provided upstreamof the electrode 30 with respect to the direction of movement L in orderto attenuate an air boundary layer carried along by the material web 20,are kept at ground or earth potential V_(E).

[0042] Between the electrode arrangement 30 and its surroundings, keptat ground potential, not orlv does an electric field build up butdischarges occur between the needle points or needle electrodes 32 andthe surroundings, which are indicated in FIG. 1 by the dashed lines E.Because of these discharges E, the application medium 16 of the curtain18 is charged up electrically, so that, in the electric field betweenthe electrode arrangement 30 and the supporting roll 22, it isaccelerated toward the supporting roll 22, which assists thepre-stretching.

[0043] Furthermore, however, because of the discharges E, a force isalso exerted on the curtain 18 which has a component extendingorthogonally to the falling movement direction of said curtain. Thisforce component also helps to stabilize the application medium curtain18 against the influence of the air boundary layer G carried along onthe surface of the material web 20.

[0044] As already mentioned above, the suction box 34 is used toattenuate the air boundary layer G. In order to increase the suctionefficiency, a trailing scraper 36 is provided on the outlet side of thissuction box 34, which is in sliding contact with the surface 20 a of thematerial web 20 and seals off the suction area of the suction box 34 onthe outlet side. The trailing scraper 36 can be fabricated, for example,from a composite material coated with Teflon. Because of the use of acomposite material, the trailing scraper 36 has adequate temperatureresistance and flexibility and, because of the surface coating withTeflon, has an adequately low friction.

[0045] According to the invention, a further electrode arrangement 40 isnow provided between this trailing scraper 36 and the suction box 34,and is fitted to the suction box 34 via an electrical insulator 42. Thefurther electrode arrangement 40 can again be formed either by a flatelectrode having a plurality of needle points or a plurality of needleelectrodes arranged adjacent to one another in the transverse directionQ. In principle, the electrode arrangement 40 can be connected to anexternal voltage supply. However, in the exemplary embodimentillustrated, it is in a floating state with regard to its electricpotential (“floating potential”), but, because of the discharges Eoriginating from the electrode arrangement 30, it is charged up andtherefore likewise brought to a potential different from groundpotential.

[0046] Because of the short distance of the points of the electrodearrangement 40 from the substrate U, the charging up of the electrodearrangement 40 is sufficient to permit discharges e likewise to occurbetween the points of the electrode arrangement 40 and the substrate U.These discharges e disrupt the laminar flow of the air boundary layer Gand convert said flow, at least partly, into a turbulent flow. Thismakes it easier to suck the air boundary layer G away from the surface20 a of the material web and therefore improves the effectiveness of thesuction box 34.

[0047] Finally, electrical forces can also further contribute toimproving the equalization and fixing of the application layer 24 on thematerial web 20. For this purpose, for example, a further electrodearrangement in the form of a plate electrode 46 can be provided, whichis kept at a predetermined electric potential V₄. Between this plateelectrode 46 and the supporting roll 22 kept at ground potential thereforms a relatively homogeneous electric field, which exerts on theapplication medium a force directed toward the substrate U. At locationson the material web surface 20 a to which too much application medium 16has been applied, this force ensures the displacement of the excessapplication medium and therefore evening of the application layer 24. Inthe extreme case, it is even possible for regions of the material websurface 20 a that have hitherto not been covered to be covered withapplication medium 16 for the first time as a result of this force.Furthermore, the force mentioned above also improves the bond whichincorporates the application medium 16 with the surface 20 a of thematerial web 20.

[0048] It should further be noted that the charging which theapplication medium 16 experiences in the region of the curtain 18 as aresult of the discharges E, in interaction with the supporting roll 22kept at ground potential, leads to such a force directed toward thesubstrate U. This is indicated by the arrows F in the enlargedillustration of the detail D.

[0049] The supporting roll 22 can be kept at the ground potential V_(E)in different ways. For example, the roll shaft A can be connected to awiping contact, as described in DE 197 33 333 A1, for example.Additionally or alternatively, however, a wiping contact 50 connected tothe surface 22 a of the roll 22 can also be provided. Finally, thematerial web 20 can also be kept at ground potential V_(E) via contactsformed, for example, by web guide elements 48.

[0050]FIG. 2 illustrates a view of the application device 10 taken withthe direction of view in the direction of movement L. Using thisillustration, an explanation will be given of a possible way in whichthe application medium curtain 18 can also be stabilized in the regionof its side edges 18 a. This is because the surface tension of theapplication medium 16 which attempts to contract the curtain 18 actsprimarily in the edge regions 18 of the curtain 18. There, it leads tolateral contraction of the curtain 18 and thickening of the latter. Inorder to counteract this effect, in the embodiment illustrated in FIG.2, guide strips 54 are provided which are fitted to the applicator unit12 in such a way that they catch the curtain 18 emerging from thedischarge nozzle 14 and guide it on its falling path until immediatelybefore the surface 20 a of the material web 20.

[0051] The action of the edge strips 54 is based on the adhesion forcesbetween the application medium 16 and the surface of the edge strips 54.With regard to the best possible adhesion, the edge angle or wettingangle a of a droplet T of application medium 16 on the surface 54 a ofthe edge strips 54 should have the lowest possible value. This can beachieved, for example, by the surface of the edge strips 54 beingproduced from a material with a high surface tension. Furthermore, thewetting between application medium 16 and edge strips 54 can be improvedby a roughened, for example toothed, surface structure of the edgestrips 54. This can be achieved, for example, by threaded rods beingused as edge strips 54.

[0052] One further possible way of being able to improve the adhesion ofthe application medium 16 at the edge strips 54 is for electrode strips56 to be provided substantially parallel to the edge strips 54, to whichstrips a predetermined electric voltage V₅ is applied. As a result, in away similar to that described above for the arrangement according toFIG. 1, an attractive force is exerted on the application medium.

[0053] In order to be able to adapt the edge strips 54 to therespectively desired working width, that is to say the respectivelypresent width of the material web 20, said edge strips 54 are arrangedon the applicator unit 12 such that they can be displaced in thetransverse direction Q, which is indicated by the arrows q in FIG. 2.Furthermore, the edge strips 54 can also be arranged on the applicatorunit 12 such they can be pivoted, specifically about axes runningsubstantially parallel to the longitudinal direction L and/or to thetransverse direction Q, the arrows 1 in FIG. 2 merely indicating theability to pivot in the transverse direction Q, that is to say about anaxis running substantially parallel to the longitudinal direction L.

1. A device (10) for applying liquid or pasty application medium (16) toone or both sides of a moving substrate (U), comprising an applicatorunit (12) which is arranged at a distance from the substrate (U) anddischarges the application medium (16) onto the substrate (U) in theform of a free application medium jet (18), the substrate (U), in thecase of direct application, being the surface (20 a) of a material web(20), in particular of paper or board, and, in the case of indirectapplication, being the surface of a transfer element, preferably of atransfer roll, which then transfers the application medium to thesurface of the material web, characterized in that in the region of theapplicator unit (12) there is provided a device (30) for producing anelectric field, which exerts on the application medium jet (18) movingfrom the applicator unit (12) to the substrate (U) a force which assistsits movement.
 2. The application device as claimed in claim 1,characterized in that the applicator unit is a curtain applicator unit(12) which discharges the application medium onto the substrate (U) asan application medium curtain (18) which moves from the applicator unit(12) to the substrate (U) substantially under the force of gravity. 3.The application device as claimed in claim 1, characterized in that theapplicator unit is a free jet nozzle applicator unit, which dischargesthe application medium onto the substrate as an application medium jetwhich moves from the applicator unit to the substrate substantially onaccount of the expulsion momentum imparted to it by the applicator unit.4. The application device as claimed in one of the preceding claims,characterized in that the applicator unit (12) is kept at a firstpredetermined electric potential (V₁).
 5. The application device asclaimed in one of the preceding claims, characterized in that, at leastin the vicinity of the applicator unit 12, the substrate (U) is kept ata second predetermined electric potential (V_(E) or V₂).
 6. Theapplication device as claimed in one of the preceding claims,characterized in that an electrode arrangement (30) is provided on theupstream side of the applicator unit (12) and in its vicinity,preferably at a distance from the substrate (U), which is kept at athird predetermined electric potential (V₃).
 7. The application deviceas claimed in claim 6, characterized in that the electrode arrangementcomprises at least one flat electrode, the flat electrode preferablyhaving a plurality of projections or needle points on its side pointingtoward the substrate.
 8. The application device as claimed in claim 6,characterized in that the electrode arrangement (30) comprises aplurality of individual electrodes, preferably needle electrodes (32),arranged adjacent to one another in the transverse direction of thesubstrate (U).
 9. The application device as claimed in one of thepreceding claims, characterized in that downstream of the position (P)at which the application medium (16) strikes the substrate there isprovided a further device (46) for producing an electric field, whichexerts on the application medium (16) applied to the substrate (U) aforce which is directed toward the substrate (U).
 10. The applicationdevice as claimed in claim 9, characterized in that the further fieldproduction device has a further electrode arrangement (46) which isadjacent to the substrate (U) and which is preferably kept at a fourthpredetermined electric potential (V₄).
 11. The application device asclaimed in one of claims 4 to 10, characterized in that the first and/orthe third and/or the fourth predetermined electric potential (V₁, V₃,V₄) have a value of between about 5 kV and about 60 kV, preferably about30 kV.
 12. The application device as claimed in one of claims 5 to 11,characterized in that the second predetermined electric potential (V₂)is the ground potential (V_(E)).
 13. The application device as claimedin claim 12, characterized in that a backing element, preferably abacking roll (22) which, in the case of direct application, supports thematerial web (20) in the region of the applicator unit (12) or, in thecase of indirect application, on the surface of which the applicatorunit applies the application medium, is in contact with an electrode inorder to keep said roll at the second predetermined electric potential(V₂).
 14. The application device as claimed in claim 13, characterizedin that the surface (22 a) of the backing element (22) is in wipingcontact with the electrode (50).
 15. The application device as claimedin claim 13 or 14, characterized in that the electrode is in contactwith a bearing shaft (A) of the backing roll (22).
 16. The applicationdevice as claimed in one of claims 12 to 15, characterized in that, inthe case of direct application, the material web (20) is kept at thesecond predetermined electric potential (V₂) by an electrode (48)formed, for example, as a web guide element.
 17. The application deviceas claimed in one of the preceding claims, characterized in that, inaddition to or instead of the electrode arrangement (30) and/or thefurther electrode arrangement (46), a magnetic field device is providedfor influencing the movement of the application medium (16).
 18. Theapplication device as claimed in one of the preceding claims,characterized in that, in the direction of movement (L) of the substrate(U), a device (34) for attenuating the air boundary layer (G) carriedalong by the substrate (U) is arranged upstream of the electrodearrangement (30).
 19. The application device as claimed in claim 18,characterized in that the air boundary layer attenuation devicecomprises a suction device (34).
 20. The application device as claimedin claim 19, characterized in that a trailing scraper (36) in wipingcontact with the substrate (U) is provided at the downstream end of thesuction device (34).
 21. The application device as claimed in one ofclaims 18 to 20, characterized in that a further electrode arrangement(40) is provided in the region of the air boundary layer attenuationdevice (34), preferably between the downstream end of the suction device(34) and the trailing scraper (36).
 22. The application device asclaimed in claim 21, characterized in that the further electrodearrangement (40) comprises a plurality of individual electrodes,preferably needle electrodes, arranged adjacent to one another in thetransverse direction of the substrate.
 23. The application device asclaimed in claim 21, characterized in that the further electrodearrangement (40) comprises at least one flat electrode which has aplurality of projections or needle points on its side pointing towardthe substrate.
 24. The application device as claimed in one of claims 21to 23, characterized in that the further electrode arrangement (40) hasa distance from the substrate of between about 2 mm and about 30 mm. 25.The application device as claimed in one of claims 21 to 24,characterized in that the electric potential of the further electrodearrangement (40) is kept floating.
 26. The application device as claimedin one of claims 21 to 25, characterized in that the further electrodearrangement (40) is arranged on the air boundary layer attenuationdevice (34) electrically insulated (42) from the latter.
 27. Theapplication device as claimed in one of the preceding claims,characterized in that the applicator unit (12) and/or the air boundarylayer attenuation device (34) is kept at a predetermined electricpotential (V_(E)), preferably ground potential.
 28. A device (10) forapplying liquid or pasty application medium (16) to one or both sides ofa moving substrate (U), comprising a curtain applicator unit (12) whichdischarges the application medium (16) onto the substrate (U) as acurtain (18) or veil moving substantially under the force of gravity,the substrate (U), in the case of direct application, being the surface(20 a) of a material web (20), in particular of paper or board, and, inthe case of indirect application, being the surface of a transferelement, preferably of a transfer roll, which then transfers theapplication medium to the surface of the material web, characterized inthat edge guiding elements (54) are provided, which guide the lateraledges (18 a) of the application medium curtain (18) at least on part ofits movement under the force of gravity between the curtain applicatorunit (12) and the substrate (U).
 29. The application device as claimedin claim 28, characterized in that the surface characteristics of atleast one edge guiding element (54) are chosen in such a way that thewetting angle (a), which depends on the characteristics of theapplication medium (16) and the surface (54 a) of the edge guidingelements (54), is less than 90°.
 30. The application device as claimedin claim 28 or 29, characterized in that at least one edge guidingelement (54) has a structured surface.
 31. The application device asclaimed in claim 30, characterized in that the surface (54 a) of atleast one edge guiding element (54) is roughened.
 32. The applicationdevice as claimed in claim 30, characterized in that at least one edgeguiding element (54) has a toothed surface (54 a).
 33. The applicationdevice as claimed in claim 32, characterized in that at least one edgeguiding element (54) has an external thread.
 34. The application deviceas claimed in one of claims 28 to 33, characterized in that at least oneedge guiding element (54) is fabricated from glass or metal.
 35. Theapplication device as claimed in one of claims 28 to 34, characterizedin that at least one edge guiding element (54) is arranged such that itcan be displaced in the transverse direction (Q) of the substrate (U).36. The application device as claimed in one of claims 28 to 35,characterized in that the angle which an edge guiding element (54) formswith the vertical is adjustable.
 37. The application device as claimedin one of claims 28 to 36, characterized in that an electrode (56) isprovided in the vicinity of at least one of the edge guiding elements(54), preferably extending substantially parallel to the latter, and iskept at a predetermined electric potential (V₅).